LTE RACH Procedure defined as per 3GPP TS 36.300 (10.1.5) & 36.211 (5.7). Describes RACH its use, its types and calculations related to RACH power.

1. 3GPP TS 36.300 (10.1.5) & 36.211 (5.7)
Composed by –AALEKH JAIN
RACH Procedure

2. Content Coverage
Introduction RACH Pre-conditions When RACH occurs?
Types of RACHLayer Interaction RACHPRACH
PRACH Power

3. Introduction
RACH stands for Random Access Channel. This is the first message from UE to eNB when you power it on.
The main purpose of RACH can be described as follows.
i. Achieve UP link synchronization between UE and eNB
ii. Obtain the resource for Message 3 (e.g, RRC Connection Request)
In most of the communication, the most important precondition is to establish the timing synchronization
between the receiver and transmitter. In LTE, the synchronization in downlink (Transmitter = eNB, Receiver =
UE), is achieved by the special synchronization channel. This downlink sync signal gets broadcasted to
everybody and it is get transmitted all the time with a certain interval.
However in Uplink (Transmitter = UE, Receiver = eNB), it is not efficient if UE is using this kind of
broadcasting/always-on synchronization mechanism. In case of uplink, this synchronization process should meet
following criteria
i. The synchronization process should happen only when there is immediate necessity
ii. The synchronization should be dedicated to only a specific UE
Another purpose of RACH process is to obtain the resource for Msg3 (Message 3). RRC Connection Request is
one example of Msg3.

4. RACH Preconditions
Before UE decided to send RACH signal (RACH preamble), there are many preconditions to be met.
Following procedure is to list each steps from Power-On to Initial PRACH.
i. UE is Off
ii. Power On UE
iii. < Frequency Search >
iv. Time and Frame Synchronization : In this process, PSS and SSS will be decoded as well.
v. PCI (Physical Cell ID) detection
vi. MIB decoding : UE can figure out System Bandwidth and Transmission Mode in this process.
vii. Detect CSR (Cell Specific Reference Signal) and perform Channel Estimation and Equalization. In this
process, UE will detect/measure reference signal across the whole system bandwidth. So RSRP/RSRQ
measured at this step can be a good indicator for overall signal quality.
viii. Decode PDCCH and extract DCI information for SIB. PDCCH is spread across the whole bandwidth, so the
signal quality across the whole bandwidth should be good enough for this step.
ix. SIB deconding (SIB1 should be decoded first and then SIB2 and then remaining SIBs)
x. < Cell Selection > : UE may find multiple suitable cells, but it try camp on to HPLM cell with the highest
priority
xi. < Initial RACH Process >

5. When RACH occurs?
The random access procedure is performed for the following events related to the PCell:
• Initial access from RRC_IDLE
• RRC Connection Re-establishment procedure
• Handover, except for NB-IoT or when RACH-less HO is configured
• DL data arrival during RRC_CONNECTED requiring random access procedure: E.g. when UL synchronisation
status is "non-synchronised".
• UL data arrival during RRC_CONNECTED requiring random access procedure: E.g. when UL synchronisation
status is "non-synchronised" or there are no PUCCH resources for SR available.
• For positioning purpose during RRC_CONNECTED requiring random access procedure: E.g. when timing
advance is needed for UE positioning.
The random access procedure is also performed on a SCell to establish time alignment for the corresponding
sTAG. For E-UTRA connected to 5GC, the random access procedure is also performed for the transition from
RRC_INACTIVE.
In DC, the random access procedure is also performed on at least PSCell upon SCG addition/modification, if
instructed, or upon DL/UL data arrival during RRC_CONNECTED requiring random access procedure. The UE
initiated random access procedure is performed only on PSCell for SCG.

6. Types of RACH
the random access procedure takes two distinct forms:
o Contention
o Non-contention based (applicable to only handover, DL data arrival, positioning and obtaining timing
advance alignment for a sTAG)
When a UE transmit a PRACH Preamble, it transmits with a specific pattern and this specific pattern is called a
"Signature". In each LTE cell, total 64 preamble signatures are available and UE select randomly one of these
signatures.
There is some possibility that multiple UEs send PRACH with identical signatures. It means the same PRACH
preamble from multiple UE reaches the NW at the same time, this kind of PRACH collision is called Contention
and the RACH process that allows this type of Contention is called Contention based RACH Process. In this kind
of RACH process, Network would go through additional process at later step to resolve these contention and
this process is called "Contention Resolution" step.
But there is some cases that these kind of contention is not acceptable. Usually in this case, the Network
informs each of the UE of exactly when and which preamble signature it has to use. Of course, in this case
Network will allocate these preamble signature so that it would not collide. This kind of RACH process is called
Non-Contention RACH procedure. To initiate the "Contention Free" RACH process, UE should be in Connected
Mode before the RACH process as in Handover case.

7. Contention based RACH
1) Random Access Preamble on RACH in uplink:
RA-RNTI, indication for L2/L3 message size
2) Random Access Response generated by MAC on DL-SCH:
Timing Advance, T_C-RNTI, UL grant for L2/L3 message
3) First scheduled UL transmission on UL-SCH:
L2/L3 message depending on RACH occurrence
4) Contention Resolution on DL:
Early contention resolution, Addressed to:
- The Temporary C-RNTI on PDCCH for initial access and after radio
link failure;
- The C-RNTI on PDCCH for UE in RRC_CONNECTED.
HARQ feedback is transmitted only by the UE which detects its own UE
identity, as provided in message 3, echoed in the Contention Resolution
message;

8. Non-Contention based RACH
0) Random Access Preamble assignment via dedicated signalling in DL:
eNB assigns to UE a non-contention Random Access Preamble
1) Random Access Preamble on RACH in uplink:
UE transmits the assigned non-contention Random Access Preamble
2) Random Access Response on DL-SCH:
Random Access Response (Timing Advance, C-RNTI, UL grant for
L2/L3 message)
After receiving UL grant, UE sends the L2/L3 message depending on the
RACH occurrence

9. Layer Interaction RACH
Random access procedure described above is modelled below from L1 and L2/3 interaction point of view. L2/L3
receives indication from L1 whether ACK is received or DTX is detected after indication of Random Access
Preamble transmission to L1. L2/3 indicates L1 to transmit first scheduled UL transmission (RRC Connection
Request in case of initial access) if necessary or Random Access Preamble based on the indication from L1.

10. Physical RACH Preamble (PRACH)
The physical layer random access preamble, consists of a cyclic prefix of length TCP and a sequence part of
length TSEQ. UE determines which Preamble format it has to use by following table. PRACH Configuration Index
in sib2 determines the Preamble Format to be used.
Ts is defined as 1/(15000 x 2048) seconds (=0.03255 us)

11. RACH information in sib2
Used in PRACH tx
Power calculations
Decides which
preamble format
will be used
Decides preamble
groups to be used
Calculates NCS -
RACH signal cycle
shift
Used in PRACH tx
Power calculations
Used in PRACH
retransmission
Power calculations

12. PRACH Power
The RACH Preamble (PRACH) Power (P_PRACH) is determined by the following equation.
P_PRACH = min{P_CMAX, PREAMBLE_RECEIVED_TARGET_POWER + PL}
PL stands for Path Loss between eNB Tx antenna and UE Rx Antenna. PREAMBLE_RECEIVED_TARGET_POWER is
the PRACH power that eNB expect to receive.
i. Calculate PREAMBLE_RECEIVED_TARGET_POWER + PL
ii. if the calculated value is less than P_CMAX(23 dBm), transmit the PRACH at the calculated value
iii. if the calculated value is greater than P_CMAX(23 dBm), transmit the PRACH at P_CMAX
< Case 1 > When UE send the first PRACH
PREAMBLE_RECEIVED_TARGET_POWER = preambleInitialReceivedTargetPower(in SIB2) + DELTA_PREAMBLE
< Case 2 > When UE retransmit PRACH
PREAMBLE_RECEIVED_TARGET_POWER = preambleInitialReceivedTargetPower(in SIB2) + DELTA_PREAMBLE
+ (PREAMBLE_TRANSMISSION_COUNTER – 1) * powerRampingStep
PREAMBLE_TRANSMISSION_COUNTER starts from 1 at the
first PRACH and get increased by 1 every time PRACH get
retransmitted.
DELTA_PREAMBLE is determined by which Preamble Format
is used.

13. PRACH Power
PL = eNB Transmitter Power - UE Reciever Power, where eNB Transmitter Power is referenceSignalPower.
Let's take an example from above shared sib2 information
preambleInitialReceivedTargetPower = -108 (dBm)
referenceSignalPower = 21
prach-ConfigIndex: 3 ==> PREAMBLE FORMAT is Format 0.
powerRampingStep = dB4
Now assume that UE measures RSRP at its receiver antenna = -90dBm
From these information,
PREAMBLE_RECEIVED_TARGET_POWER = preambleInitialReceivedTargetPower(in SIB2) + DELTA_PREAMBLE
= -108 + 0 = -108
PL = (referenceSignalPowerin SIB2) - (RSRP measuredt at UE) = 21 - (-90) = 111
Initial P_PRACH = min{23, -108+111 } = min(23, 3) = 3 dBm
First Retransmittion P_PRACH = min{23,-108+111+(2-1)*4} = min(23,7) = 7 dBm

14. Thanks